The present invention relates generally to signal generators and more particularly a signal generator for generating jitter or noise on a selected bit in a serial digital signal output and displaying characteristics of a serial digital signal output.
Electronic instruments using microprocessors, such as a personal computer, test and measurement instruments and the like, use serial data transfer between devices for faster data transfer rate, even though a CPU processes the data in parallel. Serial transmission paths have a data transmitter side that converts the parallel data into serial data (a serial digital signal) and a data receiver side that converts the serial data into the parallel data.
The serial digital signal transfers digital data in the form of “1s” and “0s” with the digital serial signal waveform having ideal edges representing rectangular transitions in the signal. FIG. 1 shows a serial digital signal having rising and falling edges representing rectangle transitions. If the propagation path between a transmitter and a receiver is ideal, the transition edges of the waveform will not change even after the propagation and would be an ideal waveform as indicated by the dotted line. A serial digital signal may transmit one or more bits during a symbol period T according to a particular modulation method. FIG. 1 shows a one bit case. The receiver detects values of the serial digital signal every symbol period T to demodulate the “1s” and “0s” of the digital data.
As shown as solid lines in FIG. 1, an actual transferred serial digital signal usually has distortions relative to ideal serial digital signal depending on the characteristics of the propagation path, the signal speed, etc. The information transferred by the serial digital signal is “1s” and “0s” but the signal itself is an analog electric signal. A waveform display apparatus, such as a digital oscilloscope, may be used for storing waveform data of the serial digital signal to display the eye pattern for measuring the characteristics of the jitter and the like in the serial digital signal.
FIG. 2 shows an eye pattern display of waveform data acquired with a waveform display apparatus. The serial digital signal acquired as the waveform data may be a 32 bit signal transferred as packets and the eye pattern may be displayed by repetitively overlaying the signal waveforms of the respective bits. In the eye pattern display, the larger the waveform distortion is, the smaller the area size of the eye pattern becomes or the larger difference from the ideal shape it has. U.S. Pat. No. 6,806,877 discloses the invention that measures the shape or area size of an eye for evaluating the quality of the digital signal. The eye pattern display indicates the frequency of occurrences of the overlain signal waveforms using varying colors. That is, the higher the frequency of occurrence results in a color closer to the red and the lower the frequency of occurrence results in a color closer to violet. The eye pattern is also called as an eye diagram.
The serial digital signal waveform may be analyzed for digital signal quality by characterizing jitter characteristics of rising and falling edges of a waveform. The results of the jitter characteristics are displayed with known various graphs. For example, FIG. 3 is a graph of a time trend of jitter wherein the horizontal and vertical axes respectively indicate time and jitter size to show how much the jitter size changes with time. FIG. 4 is a histogram showing jitter frequency. Jitter derived from heat of a device, for example, has a histogram represented by a Gaussian curve having a peak around jitter amount zero. However, pattern dependent, i.e. “0s” and “1s” may have a histogram as shown in FIG. 4. Additional signal characteristics, such as jitter spectrums for analyzing the frequency components of the jitter, the maximum and minimum values of the jitter, etc., may be displayed as graphs and/or numeric values.
In a development process of an electronic instrument, it is required to confirm whether the circuits (circuits under test) in the electronic instrument work properly. One way is to provide signals required for the circuit operation with a signal generator and observe the signals in the circuits under test with an oscilloscope, logic analyzer or the like.
Digital circuits and transmission paths generally generate noises and/or jitter on digital signal. Then, it is desired that the signal generator generates a digital signal that reflects such noises and/or jitter. However, a conventional signal generator can not provide a digital signal that includes bit pattern dependent jitter. Therefore, if a system has circuits, and data in which a bit sequence is not repetitive, such as PRBS (Pseudorandom Binary Sequence) flowing in one circuit while and affected by other circuits, it is difficult to analyze and evaluate such a system using the signal generator.
Therefore, it is desired that a signal generator can provide a signal that jitter and/or noises are selectively added to only desired potions of the output signal.